Curable resins have long been used to insulate metallic conductors. Representative is their use as magnet wire enamels--see, for example, U.S. Pat. Nos. 2,936,296; 3,342,780; U.K. Pat. No. 973,377; U.S. Pat. No. 3,426,098; and U.S. Pat. No. 3,668,175.
Such enamels most commonly comprise a solution of the resin in a solvent therefor. This solution may be applied to the conductor, dried and subjected to conventional curing conditions. This results in the production of a solid coating or wall of insulating resin peripherally about the conductor.
Myriad resins or combinations of resins have been employed in such enamels. Polyester resins, particularly ones produced through condensation of glycols and polyfunctional acids or anhydrides, have long been utilized in this manner. See, e.g., U.S. Pat. No. 2,936,296.
In U.S. Pat. No. 3,541,038, the production of high molecular weight polyimidamide resins by condensation of tribasic acid anhydride with diisocyanate compounds is described. The resultant resins are especially useful for coil-impregnation or electrical insulation. Tough films can also be produced from solutions of them.
More recently, polyetherimides comprising the reaction products of bis ether anhydrides with organic diamines have been suggested for use as wire coatings. They can be deposited on the conductor from simple solvents, U.S. Pat. No. 3,847,867; or as powders from fluidized beds, U.S. Pat. No. 4,098,800; or from reactive ether solvents, U.S. Pat. No. 4,115,341; or as reaction products prepared in the presence of a phenolic solvent, U.S. Pat. No. 4,024,010.
Polyisocyanate compounds, preferably employed in blocked form, are also described in U.S. patent application Ser. No. 53,317 where they are reacted to cross-link various polyetherimides. The resultant resins make highly preferred enamels for electrical insulation.
It is also known to upgrade the properties of such wire enamels by including minor proportions of other additives. For example, U.S. Pat. No. 3,668,175 confirms that the addition of titanate esters, phenolic resins, and the like, have exemplary effects on such important properties of wire enamels as flexibility, abrasion resistance, heat shock, cut-through temperature, and thermal life. This patent also discloses a general improvement in heat shock, at some expense to thermal life, when adding blocked polyisocyanates to enamels comprising polyester amide imides.
Notwithstanding the foregoing and other improvements in the properties of enamels, drawbacks respecting their application have remained. Most importantly, enamels applied from solvent solution commonly produce rough and grainy coatings. Such coating walls are inefficient as insulators and result in substantial scrap losses.